1. Field of the Invention
This invention relates generally to wind turbines that operate at variable speed under varying wind speeds, and relates more particularly to a power converter for converting varying wind energy into substantially smooth AC electrical power having a controlled reactive power.
2. Description of the Relevant Art
Wind turbines provide a primary source of energy that can be converted into electricity and supplied to utility power grids. Conversion of wind energy to electrical energy is accomplished in a wind turbine by driving an electrical generator, which is commonly an AC induction generator. If the electrical power generated by a wind turbine is to be supplied to a utility power grid, then it must have a constant frequency. Conventionally, constant frequency operation is accomplished by driving the generator at a constant rotational speed which requires that the wind turbine rotate at a constant speed unless a variable speed transmission is used. Unfortunately, constant speed operation of a wind turbine limits energy conversion efficiency due to the variable wind speeds powering the wind turbine. For optimal energy recovery, turbine rotor speed is controlled as a function of pitch angle and wind speed.
Variable speed wind turbines have been proposed for increasing the energy conversion efficiencies of constant speed wind turbines. By varying the rotor speed with the varying wind speed, improved energy recovery can be achieved over a range of wind speeds. Also, mechanical stresses caused by wind gusts can be reduced by limiting the torque acting on the wind turbine by the generator and allowing the wind turbine to speed up in response to wind gusts.
Although variable speed wind turbines are advantageous from the perspective of increased energy conversion and reduced stresses, the electrical generation system is more complicated than that of a constant speed wind turbine. Since a generator is usually coupled to a variable speed rotor through a fixed-ratio gear transmission, the electrical power produced by the generator will have a variable frequency. This variable frequency AC must be converted to a constant frequency AC before it can be used by the utility power grid. The conversion can be accomplished either directly by a frequency converter or through an intermediate conversion to DC by a rectifier and subsequent inversion to fixed-frequency AC by means of an inverter.
For a utility using power generated by wind turbines, a substantial disadvantage is the power output fluctuation caused by wind speed variations. Wind-caused power fluctuations will occur using any wind turbine, whether it be variable speed or constant speed. Problems resulting from power fluctuation are particularly acute if wind turbines supply a large portion of the energy needs of the grid, such as 10% or even 25% which is common in remote installations. Power fluctuations require the utility to make up a deficit from other sources such as oil-fired generators. When a power deficit occurs, these generators may be loaded sufficiently beyond their capacity that they slow down. When they slow down, the frequency of the output changes, for example, to 55 or even 50 Hz from the standard 60 Hz frequency. Such low frequencies can adversely affect appliances such as clocks and other electrical equipment. It would be an advantage to provide a wind turbine with an approximately constant of power output regardless of the inevitable wind speed fluctuations. If power output fluctuations from wind turbines could be minimized or even avoided completely, then utility grids would not be required to supply the deficit power from their generators and a constant 60 Hz frequency could be maintained, therefore avoiding adverse effects on electrical equipment. It would be an advantage to provide a wind turbine that supplies a constant amount of power regardless of wind speed variations.
Utility companies prefer to supply their customers with power at a unity power factor, meaning that the current and voltage are exactly in phase. However, their customers' loads are often such that current tends to be pulled out of phase with the voltage. For example, a customer who has many motors operating in his facility will cause the current to lag the voltage at an angle between zero and 90.degree.. A load that tends to cause the current to become out of phase with the voltage is termed a "reactive load". Such reactive loads are common and are measured in Volt-Ampere Reactive units, or "VARs". In order to address the problem of reactive loads, utilities have relied upon banks of capacitors to help pull the current back in phase. However, capacitor banks have drawbacks including limits on the number of VARs that can be supplied to counteract inductive loads and a limited ability to select the amount of VARs to be supplied to the utility.
Conventional constant speed wind turbines provide output power with a lagging current. In order to provide a power factor of unity to the utility during operation, constant speed wind turbine plants have employed large capacitor banks to pull the current in phase with the voltage. The capacitor banks have all the disadvantages outlined above, including lack of ability to select the number of VARs necessary to precisely compensate for the VARs provided by the wind turbines. As an additional disadvantage, capacitor banks produce unpredictable switching transients when switching the capacitor banks on- or off- line.
When the wind is not blowing, a wind turbine cannot produce real power because the rotor is not turning. During periods of no wind, conventional wind turbines sit idle, doing nothing. It would be an advantage to provide a wind turbine that has a static VAR compensator with an ability to provide reactive power in the form of VARs to the utility whether or not the wind turbine is operating. It would be a further advantage if the number of VARs were selectable independent of the amount of real power flowing through the static VAR compensator. It would be an even further advantage if the static VAR compensator were an element of the wind turbine, and therefore VAR compensation would require no additional hardware such as expensive capacitor banks and switching devices.